1. Field of the Disclosure
The present invention relates to a Class D amplifier utilizing a half-bridge circuit with bi-directional HEMT/GaN transistors.
2. Related Art
The traditional half-bridge circuit, sometimes known as an inverter, is one of the most fundamental circuit switch configurations used in power electronics today. Half-bridges are commonly used in a wide variety of applications, including electronic ballasts for lighting, class-D audio amplifiers, resonant mode power supplies and motor drive circuits. The typical half-bridge circuit 10 is illustrated in FIG. 1 and includes two switches M1, M2 connected in a totem poll configuration between a DC bus and ground. The switches are typically MOSFETsS or IGBTs.
The drain of the upper (high side) MOSFET M1 in FIG. 1 is connected to the DC bus voltage which is generally several hundreds of volts. If M1 were an IGBT, the collector would be connected to the DC bus voltage. The source of the second (low side) MOSFET M2 in FIG. 1 is connected to ground. Two free flowing diodes D1 and D2 are placed in parallel with the switches M1, M2, respectively, with the cathode connected to the drain and the anode connected to the source. In this manner, the half-bridge is formed and converts the DC bus voltage into a square-wave voltage. A half-bridge control circuit 12 controls the on-off times of each of the switches M1, M2. The circuit 12 provides the correct timing of the voltages at the gates of the switches M1, M2 to set a desired frequency, duty cycle and dead time for the square wave. The mid-point node VS of the half-bridge is typically connected to a resonant type load circuit (L1, RLOAD, C1, C2). A snubber capacitor CSNUB is preferably connected between node VS and ground, or between VS and the DC bus, to control the dv/dt of the rising and falling edges of the square wave. The diodes D1, D2 provide a current path during the dead time when neither switch is active to maintain a continuous current through the load.
When M1 turns ON, current flows from the DC bus, through M1 and through the load circuit to ground. When M1 turns off, current continues to flow out of the snubber capacitor CSNUB through the load to ground. As current flows out of the capacitor CSNUB, the voltage at node VS discharges linearly to ground at a rate provided by the following equation:dv/dt=I*CSNUB.When the voltage at VS reaches 0.7 V below ground, the lower diode D2 will become forward biased and current will continue to flow from ground through diode D2 and through the load back to ground. At some point after the voltage at VS reaches ground, the dead time will end and the switch M2 will turn ON. The load current will transition from diode D2 to the channel of switch M2. When the load current changes direction due to the resonant nature of the circuit, the current will continue to flow from the load through the switch M2 and to ground. When M2 is turned OFF, the load current will flow into CSNUB and charge the node VS up linearly at a rate defined by the equation:dv/dt=I*CSNUB.When the voltage at node VS reaches 0.7 volts above the DC bus voltage, the diode D1 will become forward biased and current will flow from ground, through the load, through D1 to the DC bus and through capacitors C1, C2 and back to ground. When the dead time is over, M1 will turn on and the cycle repeats. Thus, using the MOSFETs and the diodes D1, D2 allows for smooth and continuous current in both positive and negative directions.
When the half-bridge 10 is connected to an AC line voltage source 14, which serves and an input voltage, as illustrated in FIG. 2, for example, it is necessary to provide a bridge rectifier 4 to allow for bi-directional current flow from the AC line input voltage. FIG. 3A illustrates the AC line voltage. FIG. 3B illustrates the rectified voltage provided by the rectifier bridge 4. FIG. 3C illustrates the voltage at the node VS.
The rectifier bridge 4 keeps the voltage at the drain of the high side MOSFET M1 (or IGBT) always equal to, or above ground. This is important since if the DC bus voltage goes negative, the diodes D1 and D2 will be forward biased at the same time and a short circuit will result. The short circuit will result in the square wave becoming non-functional while the high current damages circuit components. Utilizing the bridge 4, however, the voltage at node VS will transition between the DC bus voltage and ground continuously at a certain frequency and duty cycle, as is illustrated in FIG. 3C. The amplitude of the square wave is given by the DC bus voltage level. If the capacitors C1, C2 are small, the amplitude follows the peak rectified voltage as shown in FIG. 3C.
Where MOSFETs are used, the channel allows for current to flow bi-directionally, however, the diodes D1 and D2 are inherent to the devices and cannot be removed. If IGBT switches are used, the switches conduct in only one direction, from the collector to the emitter, and thus, diodes must be added to provide bi-directional current flow.
Accordingly, it would be beneficial to provide a half-bridge circuit that allows bi-directional current flow without the need for the additional diodes described above.